As well known, SYNCHROPHASER.RTM. phase control devices (SYNCHROPHASER is a registered trademark of United Technologies Corporation) are used to maintain a relative phase relationship between propellers on multi-engine aircraft. The relative phase value is selected for each aircraft, and is maintained at a near constant value during flight to minimize acoustic cabin noise resulting from random striking of the fuselage by airflow disturbances produced by asynchronous propeller rotation.
The prior art SYNCHROPHASER phase controls use a master/slave protocol. The phase control maintains the instantaneous spatial position of the blades of each slave engine propeller in dependence on the sensed blade position of a designated master engine propeller. The sensed position of each slave propeller is compared with the sensed position of the master propeller to detect a difference phase value. The blade pitch angle of each slave propeller is then adjusted to establish the desired phase difference value.
Phase synchronization is achieved by a momentary (or possibly fixed) change in the slave propeller shaft speed. As known, the shaft speed of a propeller may be changed by changing the propeller blade pitch angle in the presence of a constant engine power level. An increase in the blade pitch angle increases the blade power absorption and reduces propeller speed. A decreases in blade pitch angle reduces power absorption and increases propeller speed. The phase control changes the blade pitch angle constantly, making constant adjustments in the shaft speed of each slave engine to maintain phase synchronization of each slave propeller with the master propeller.
The master/slave protocol, while acceptable to small business aircraft manufacturers (commuter or business), is not permitted by some of the large commercial aircraft manufacturers since to some it is viewed as a "cross coupling of engine functions". In the master/slave SYNCHROPHASER control scheme there is the inherent possibility of engine faults (including those not related to the SYNCHROPHASER control) in a reduced engine power direction. In the event of master engine degradation the SYNCHROPHASER control may pull down performance of the slave engines.
Although all such phase controls have only a limited control range of authority within which pull down may occur (typically +/-4.0 % of set speed), this is still considered as a significant reduction in the margin of safety for the approach and landing flight modes. It is desirable, therefore, to perform the phase synchronization function without this interdependency between slave and master engines.